15134
J. Phys. Chem. C 2008, 112, 15134–15139
Mechanism of Visible Light Photocatalytic Oxidation of Methanol in Aerated Aqueous Suspensions of Carbon-Doped TiO2 Sara Goldstein, David Behar, and Joseph Rabani* Institute of Chemistry and the Accelerator Laboratory, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed: May 6, 2008; ReVised Manuscript ReceiVed: July 21, 2008
Visible photolysis of aerated carbon-doped TiO2 (C-TiO2) aqueous suspensions induces methanol oxidation to formaldehyde. The rate of HCHO formation increases with the concentration of CH3OH or C-TiO2 and is nearly doubled in the presence of catalase or excess of H2O2. The mechanism involves oxidation of CH3OH by surface trapped holes, although these holes have lower energy than those formed upon UV photolysis of undoped TiO2. The C-TiO2 electrons reduce O2 to H2O2. CH3OH oxidation via reduction of H2O2 by the C-TiO2 electrons was observed in the presence of added H2O2, where it competes efficiently with O2 for the C-TiO2 electrons. At [H2O2] > 0.1 mM, the yield of HCHO is about twice that in the absence of added H2O2. Light absorption measurements in an integrating sphere show that the limiting absorption fraction at high [C-TiO2] is 0.5 at 450 nm, the rest of the light being mostly scattered backward. The HCHO quantum yield depends on the square root of the absorbed light density and is only a few percent at the intensities used in this work. A parallel photocatalytic decomposition of H2O2 has been observed, which is not associated with HCHO formation. O2 · - + O2 · - + 2H+ f H2O2 + O2
Introduction It is well-known that TiO2 produce conduction band electrons (eCB-) and valence band holes (hVB+) upon photolysis at λ < 388 nm, which become quickly trapped at the surface (eT- and hT+, respectively).1-3 Recently, Salvador4 unequivocally identified the reactive holes in water suspensions of TiO2 particles as surface -O · - (or - · OH depending on pH) covalently linked to Ti atoms, which are produced according to reaction 8.
hVB+ + {Ti - O2- - Ti} f {Ti - · O- - Ti}
(1)
Most organic compounds are oxidized by the holes according to reactions 2, or both. In the presence of oxygen, · RH is converted to the respective peroxyl radical (reaction 4), which may decompose via a unimolecular process (reaction 5), bimolecularly (reaction 6), or both.5
hVB+ + RH2 f · RH + H+
(2)
hT+ + RH2 f · RH + H+
(3)
· · ·
RH + O2 f · OORH
(4)
OORH f R + H+ + O2 · -
(5)
OORH + · OORH f HR - O - O - O - O - RH (6)
The TiO2 electrons are known to reduce O2 according to reactions 7-9, where e- represents eCB-, eT-, or both.
e- + O2 f O2 · -
·-
+
e + O2 (+2H ) f H2O2
(7) (8)
* To whom correspondence should be addressed. Tel: 972-2-6585292. Fax: 972-2-6586925. E-mail:
[email protected].
(9)
Subsequently, H2O2, if it accumulates to sufficient levels, is reduced by e- to form · OH (reaction 10) generating additional · RH (reaction 11).
e- + H2O2 + H+ f H2O + · OH ·
OH + RH2 f H2O + · RH
(10) (11)
The discovery that doping of TiO2 leads to extension of the photoactive region from UV to visible light has remarkably increased the interest in such doped TiO2 because of the potential application for visible light driven solar conversion6-14 and photocatalytic oxidations of organic pollutants.15 The visible photoactivity is attributed either to introduction of intragap localized states by the dopants9,16-18 or to narrowing of the band gap.19-23 Although many works report photocatalytic activity under visible illumination, including degradation of inorganic and organic compounds,8,9,23-29 the nature of the oxidation mechanism is ambiguous. The present paper concerns the photooxidation mechanism mediated by carbon-doped TiO2 (C-TiO2). Previous studies on C-TiO2 show band gap narrowing of 0.05-0.14 eV and a wide absorption in the visible range attributed to a variety of surface states.6 The C doping involves substitution of O atoms by C producing new energy states deep in the TiO2 band gap, which are responsible for the visible light absorption.19,30 Theoretical and experimental results suggest the existence of such states near to the valence band.6,19 While facilitating photocatalytic activity in the visible, the localized C 2p states act also as recombination centers for holes and electrons.31,32 However, it is not clear whether the localized holes formed upon visible light excitation of C-TiO2 oxidize organic molecules according to reactions 3-6 or the oxidation is limited to specific organic solutes where one-electrontransfer processes are involved.31 Photocatalytic oxidation
10.1021/jp803974a CCC: $40.75 2008 American Chemical Society Published on Web 09/04/2008
Photocatalytic Oxidation of Methanol
J. Phys. Chem. C, Vol. 112, No. 39, 2008 15135
has also been suggested by · OH, which is produced via O2 reduction (reactions 7-11).6 In this case, H2O2 is produced as an intermediate and its subsequent reduction by e- depends on the efficiency of reaction 10 relative to 7 and 8. The present work is aimed at studying these issues using CH3OH as a model compound. It will be shown that oxidation of CH3OH by the carbon surface holes takes place and is followed by reactions 4-6. Since oxidation of CH3OH to · CH OH is known to be relatively slow compared to other 2 aliphatic compounds,33 our results imply that the oxidation should not be limited to a small number of specific organic compounds. Experimental Section All materials were of analytical grade and were used as received. C-TiO2 (VLP 7000) and TiO2 (UVLP 7500) were obtained as a gift from Kronos Titan GmbH. Catalase solution (EC 1.11.1.6, 2 mg mL-1, 130 000 U mL-1) was purchased from Boehringer-Mannheim. The concentration of formaldehyde was determined using the Nash reagent (2 M ammonium acetate, 0.05 M acetic acid, 0.02 M acetylacetone).34 The reagent was mixed with an equal volume of the suspension, and after 10 min incubation at 60 °C, the titanium dioxide was removed by centrifugation and the concentration of formaldehyde determined from its absorption using ε412 ) 7600 M-1 cm-1 against the same suspension kept in the dark. The molar extinction coefficient obtained after the removal of the C-TiO2 was similar to that in water. The concentrations of H2O2 were determined by the molybdate-activated iodide assay35 by adding the reagents to the suspensions followed by removal of titanium dioxide. The molar extinction coefficient of I3- as determined in aqueous suspensions of 0.75 g L-1 C-TiO2 containing 0-80 µM H2O2 was 26 400 M-1 cm-1. Photolysis was carried out using the SX-17MV Applied Photophysics setup composed of Osram 150-W, ozone-free Xe lamp and a monochromator. The monochromator was set at 450 or 500 nm with slits wide open ((23.5 nm). The incident light intensity was measured with a calibrated Si photosensor (Hamamatsu S2281) coupled with the Keithley 617 programmable electrometer. Unless otherwise stated, the incident light intensity was 6.0 × 10-9 einstein s-1. Photolysis was carried out in a cylindrical Suprasil quartz cell (2-cm i.d., 2-cm length, V ) 6.4 mL) with flat windows under magnetic stirring at room temperature. The cross section of the light beam at the cell front window was ∼1.2 cm2. The illumination cell was shielded from room light. In some experiments, the light intensity was varied by introducing appropriate neutral density filters. Spectral measurements of C-TiO2 and TiO2 aqueous suspensions were carried out in an integrating sphere, Labsphere, with 15-cm diameter. The suspension in a 1-cm, four-windows cuvette was placed inside the integrating sphere at the center, and the nonabsorbed light intensity was measured with the calibrated photosensor. The monochromator was set at selected wavelengths using narrow slits ((2.3 nm).The absorbance measurements was occasionally repeated in order to verify that no precipitation occurred during the measurements since no stirring was possible inside the integrating sphere. Results The spectra of C-TiO2 and TiO2 are presented in Figure 1. The C-TiO2 spectrum is similar to that reported earlier31,32 showing a significant absorption in the visible range, whereas TiO2 has a relatively small visible absorption.
Figure 1. Spectra of C-TiO2 and TiO2. Absorption measurements of 1 g L-1 suspensions in 1-cm cuvette in an integrating sphere.
Figure 2. Absorbance measured at 450 nm as a function of C-TiO2 concentration.
Figure 3. HCHO formation by visible photocatalysis. Broadband illumination at 450 nm of aerated C-TiO2 (0.77 g L-1) suspensions in the presence of 1 mM phosphate buffer (pH 7) and 0.2 M CH3OH or 1 M CH3OH (O).
Illumination of aerated suspensions of C-TiO2 containing CH3OH shows a linear buildup of HCHO with the exposure time (Figure 3), indicating that HCHO does not react with the transient species or with other products, which may compete for the oxidizing species. The rate of HCHO formation (RHCHO) increases upon increasing [CH3OH] reaching a plateau value at 2-3 M (Figure 4). The plateau value corresponds to ΦHCHO ∼ 0.012 calculated using A450 ) 0.16 for 0.75 g L-1 C-TiO2 in 2-cm-long cell (data taken from Figure 2). A similar effect was previously observed upon UV excitation of undoped TiO2 layer, although the quantum yields and absorbed light densities in both systems are considerably different.36
15136 J. Phys. Chem. C, Vol. 112, No. 39, 2008
Figure 4. Effect of [CH3OH] on RHCHO. Broadband illumination at 450 nm of aerated suspensions containing 0.75 g L-1 C-TiO2 and 1 mM phosphate buffer (pH 7).
Figure 5. Effect of [C-TiO2] on RHCHO. Broadband illumination at 450 nm of aerated suspensions containing 1 M CH3OH and 1 mM phosphate buffer (pH 7). The solid line was drawn for convenience.
TABLE 1: Effect of Catalase on RHCHO upon Broadband Illumination at 450 nm [C-TiO2] (g L-1)
[CH3OH] (M)
[catalase] (U mL-1)
RHCHO (µM min-1)
0.75 0.75 0.76 0.76 0.76 0.76 0.73 0.73 5.0 5.0
0.2 0.2 1.0 1.0 1.0 1.0 2.0 2.0 0.2 0.2
0 14 0 80 225 525 0 88 0 110
0.11 ( 0.01 0.22 ( 0.02 0.17 ( 0.01 0.29 ( 0.01 0.31 ( 0.02 0.30 ( 0.02 0.21 ( 0.02 0.39 ( 0.02 0.23 ( 0.02 0.44 ( 0.03
RHCHO also increases with increasing [C-TiO2] (Figure 5). A plateau value could not be reached because the quartz window of the cell becomes opaque at high [C-TiO2]. Addition of catalase nearly doubles RHCHO at different [CH3OH]. The doubling is hardly dependent on the concentration of catalase as shown in Table 1. The absorbance of C-TiO2 increases with its concentration up to A450 ∼ 0.3 as shown in Figure 2. The apparent deviation from Baer’s law is caused by light scattering. The plateau is observed when practically all the light other than the backscattered is absorbed by the TiO2. The loss of ∼50% of the light includes the reflection of the light from the cell window (∼4%) and the backscattering (46%). RHCHO increases upon addition of H2O2 and is nearly doubled at [H2O2] > 0.1 mM (Figure 6), and this effect was found to be
Goldstein et al.
Figure 6. Effect of added H2O2 on RHCHO upon broadband illumination at 450 nm. Aerated suspensions of 0.75 or 5 g L-1 C-TiO2 contained 0.2 M CH3OH and 1 mM phosphate buffer at pH 7.
Figure 7. Dependence of 1/ΦHCHO on the square root of the absorbed light density upon broadband illumination at 450 nm. Aerated suspensions of 4.3 g L-1 C-TiO2 contained 2 M CH3OH and 1 mM phosphate buffer at pH 7.
independent of C-TiO2 concentration. The effect of relatively high [H2O2] (
